The chrominance signal for each line of a television picture, comprises a desirable color component and an undesirable noise component. The noise component has a stochastic spectral distribution superimposed across the color component. In the past, "high quality filters", or "H.Q. filters", have been used to eliminate some of the noise components. H.Q. filters work by generating a difference signal, whose frequency and amplitude are derived from the noise component of a line and the correlation between the color component of the line and the color component of vertical preceding lines. A digital filter having recursive and transversal signal-processing operations is used to create a noise-reduced signal by subtracting the difference signal, weighted by a correction factor, from the chrominance signal of a line.
Such H.Q. filters work fairly well if chrominance signals of vertically successive lines are well correlated. The reason being that the frequency and amplitude of the difference signal would be determined essentially by the noise component of the line to be processed. The result being a difference signal with a low frequency and small amplitude, that is inputted to the recursive digital filter without modification.
However, H.Q. filters do not work well if the chrominance signals of vertically successive lines are not suitably correlated. Such a scenario occurs whenever there is a sudden color transition, in a television picture, between two lines that follow each other closely. In such an event, the difference signal created has a high-frequency and a large signal amplitude. If this difference signal is applied to the input of the recursive digital filter, blurring of the color transitions would occur. This effect, known as "color hanging", impairs the color fidelity of the television picture. In "color hanging", the difference signal applied to the recursive signal-processing circuitry results in a reference chrominance signal that is no longer determined primarily by the noise component to be eliminated. The reference chrominance signal is now determined by the color transition, which occurred in the vertically preceding lines, so that an efficient reduction or elimination of the color noise is no longer possible.
The analog circuitry used in the H.Q. filter method is relatively costly and complicated. Furthermore, if a PAL color television signal were to be processed, the phase of the R-Y component of a PAL chrominance signal would be reversed on alternate lines. Consequently, two delay lines are necessary in known H.Q. filters. Such delay lines are extremely expensive and serve to inflate the cost of a typical television receiver.
Another disadvantage of prior art arrangements is that they do not permit the correction of specific color errors in the television picture if used with a video recorder. During the recording of color-television signals, the lines of a field are displaced, relative to each other, on the video tape from track to track. In some playback modes, such as the search mode, the video head transcribes a slanted path across the tracks. As such, half-line displacements and sign errors of the PAL R-Y component may occur in the reproduced video signal. To correct such disturbances in the television picture, a special circuit designed for this purpose was employed. This increases the manufacturing costs of the video recorder, since the recorder must contain a circuit for correcting color noise and another circuit for eliminating the color errors just described.
The present invention discloses a method and apparatus that prevents color hanging from occurring in the presence of vertically uncorrelated chrominance signals. The apparatus, according to this invention, operates so that with only simple additional circuitry, correction of half-line displacements and phase errors of a PAL color-television signal is achieved.
The method and apparatus of the present invention creates a difference signal that is passed through a limiter. The limiter has a transfer function selected so that small difference signals propagate through the limiter unchanged. Large difference signals are suppressed by the limiter and with a transition taking place for intermediate difference signals. A difference signal, representing uncorrelated chrominance signals of vertically successive lines, is thus attenuated as a function of amplitude. A difference signal representing correlated chrominance signals of vertically successive lines is transmitted unattenuated. The modified difference signal thus obtained, is applied to the recursive and transversal signal-processing circuitry instead of the original difference signal.
One advantage of the invention over prior art arrangements is its particularly simple design. The prior art arrangements require at least two multipliers, three subtracters, and six registers.
The preferred arrangement of the present invention enables one to carry out the method according to the invention with only one multiplier, one subtracter, and three registers. This considerably simpler circuitry reduces the manufacturing costs considerably.
Prior art that discloses pertinent noise signal reduction means are exemplified by U.S. Pat. No. 4,926,361 to Ohtsubo, et al and European Patent No. 194,759 to Faroudja.
It is therefore an object of the invention to provide an arrangement and apparatus for reducing color noise signal components from television picture chrominance signals.
It is a particular object to prevent color hanging from occurring on a television screen in the presence of vertically uncorrelated chrominance signals.